[Footnote 164: _Energy_ is now defined as "_the power of doing work_," that is, the power, in _virtue of its position_ (as a head of water, a raised mass, a coiled spring) or in _virtue of its motion_ (as a falling mass, a current of wind, a projectile), to do work. The first is called _Potential_, the second _Kinetic_ Energy. Besides these instances of _Visible_ Energy, there is also _Invisible_ Molecular Energy, divided into, (_a_) the Energy of electricity in motion; (_b_) the Energy of radiant heat and light; (_c_) the kinetic Energy of absorbed heat; (_d_) molecular potential Energy; (_e_) potential Energy caused by electrical separation; (_f_) potential Energy caused by chemical separation. Of these different _kinds_ of Energy, the most available for work is Mechanical Energy, or Energy of visible motions and positions; the least available is universal heat, or radiant Energy.] [Footnote 165: See article "Energy," in _North British Review_, May, 1864, and Balfour Stewart's "Treatise on Heat," p. 370.] [Footnote 166: Stewart's "Elements of Physics," p. 357.] [Footnote 167: "Die Naturkräfte in ihrer Wechselbeziehung," p. 89.] [Footnote 168: "Correlation of Physical Forces," p. 187.] [Footnote 169: _American Journal of Science_, July, 1864.] [Footnote 170: Argyll, "Reign of Law," p. 121.] [Footnote 171: Sir Isaac Newton entertained a similar opinion. "We may be able," he said, "to form some rude conception of the _creation of matter_, if we suppose that God by his power had prevented the entrance of any thing into a certain portion of pure space which is of its nature penetrable,... from henceforward this portion of space will be endowed with _impenetrability_, one of the essential qualities of matter; and as pure space is absolutely uniform, we have only to suppose that God communicated the same impenetrability to another portion of space, and we should obtain in a certain sort the notion of _mobility_, another quality which is essential to matter."--M. Coste, Note in the 4th Edition of his "French Translation of Locke's Essay." (M. Coste reports the above from Newton's lips.)] [Footnote 172: Prof. Maxwell, in _Nature_, vol. ii. p. 219.] [Footnote 173: M. Claude Bernard, _Revue des Deux Mondes_, 1867.] [Footnote 174: "Dissertation on the Study of Natural Philosophy," § 28.] [Footnote 175: Prof. Clerk Maxwell, F.R.S., "Lecture delivered before the British Association at Bradford," in _Nature_, vol. viii. p. 441.] [Footnote 176: Müller, "Christian Doctrine of Sin," vol. i. p. 28.] [Footnote 177: "Logic," vol. ii. p. 527, 4th edition.] [Footnote 178: "Mind and Brain," vol. i. pp. 107-8.] [Footnote 179: "Mind and Brain," vol. i. p. 261.] [Footnote 180: _Nature_, vol. iv. p. 270.] [Footnote 181: See Murphy, "Habit and Intelligence," vol. i. p. 121.] [Footnote 182: Müller, "Christian Doctrine of Sin," vol. ii. p. 146.]